Composite coating compositions comprising a colored or pigmented base coat applied to a substrate and a clear coat applied on top of the base coat have become very popular as original finishes for automobiles. The color-plus-clear systems have outstanding gloss and distinctness of image. The clear coat is particularly important for these properties.
Most coating compositions that are used as clear coats in color-plus-clear composite coating systems employ hydroxyl-aminoplast cure mechanisms, well known in coating technology. The popularity of hydroxyl-aminoplast cure coating compositions is due to their low cost, durability, and attractive appearance, but it is widely recognized that such coatings, particularly clear coats, have poor resistance to etching by acid. Acid etch resistance in coatings is becoming an increasingly desirable property, particularly for automotive coatings. High solids aminoplast cured coating systems of the prior art are not highly effective for providing protection against etching caused by acid rain.
Parekh in U.S. Pat. No. 4,543,276, discloses coating compositions containing urethane compounds and any of a number of polymers having any of various functional groups including carbamate. The reference does not disclose the use of such coating compositions as clear coats in color-plus-clear composite coatings, nor does the reference teach any advantage of carbamate functional polymers in coating compositions to improve acid etch resistance.
Nordstrom in U.S. Pat. No. 3,479,328, generally discloses carbamate functional polymers which are modified with aldehyde (alkylolated) and optionally etherified, for use in coating compositions with conventional aminoplast crosslinking agents. The reference does not disclose the use of such coating compositions as clear coats in color-plus-clear composite coatings, nor does the reference teach or suggest any advantage of these polymers in coating compositions to improve acid etch resistance.